A systematic literature review on bad smells — 5 W's: which, when, what, who, where

Bad smells are sub-optimal code structures that may represent problems that need attention. We conduct an extensive literature review on a huge body of knowledge from 1990 to 2017. We show that some smells are much more studied in the literature than others and some of them are intrinsically inter-related (which). We give a perspective on how the research has been driven across time (when). In particular, while the interest in duplicated code emerged before the reference publications by Fowler and Beck and by Brown et al., other types of bad smells started to be studied only after these seminal publications, with an increasing trend in the last decade. We analyzed aims, findings, and respective experimental settings and observed that the variability on these elements may be responsible for some apparently contradictory findings on bad smells (what). Moreover, while in general bad smells of different types are studied together, only a small percentage of these studies actually investigate the relations between them (co-studies). In addition, only few relations between some types of bad smells were investigated, while there are other possible relations for further investigation. We also noted that authors have different levels of interest in the subject, some of them publishing sporadically and others continuously (who). We observed that scientific connections are ruled by a large “small world” connected graph among researchers and several small disconnected graphs. We also found that the communities studying duplicated code and other types of bad smells are largely separated. Finally, we observed that some venues are more likely to disseminate knowledge on Duplicate Code (which often is listed as a conference topic on its own), while others have a more balanced distribution among other smells (where). Finally, we provide a discussion on future directions for bad smell research.